INSIGNA is a tube equalizer in a 500 series package, using a dual triode circuit with a 12AX7 tube.
a coloring / shaping eq
with 3 bands, plus
Hi cut and Low cut filters
The eq has 24 db/oct high pass and low pass filters each with 7 frequencies.
The top and bottom band are shelving each with 8 frequencies
The mid band is peaking with 8 frequencies.
The three bands work by using buffered RC circuits in the negative feedback path around the tube amplifiers. The result is a fat sounding eq rich in second harmonic triode tube color.